As known conventionally, a metered inhaler is a device for the treatment of asthma. The dry powder inhaler (DPI) and the metered dose inhaler (MDI) are known examples of such inhalers. The metered dose inhaler is also known as a pressurized metered dose inhaler (pMDI).
Generally, the metered dose inhaler includes an aerosol canister containing medication. The aerosol canister generally includes a canister body, a valve stem extending from the canister body, and a spring installed in the canister body. The valve stem is urged to close by the spring, and releases an aerosol when depressed against the force of the spring. The valve stem is commonly held by the aerosol canister attached to the metered dose inhaler. A patient manually depresses the bottom of the canister body to push the valve stem into the canister body. The medicament-containing aerosol released from the valve stem travels through a channel inside the housing body to be ejected out of the mouthpiece attached to the housing body.
Metered dose inhalers provided with a dose counter for displaying the number of doses are widely known. The dose counter is either electronic (for example, reference 1) or mechanical (for example, References 2 and 3). In terms of cost, the mechanical dose counter is more advantageous than the expensive, electronic dose counter. The mechanical dose counter is also advantageous to provide disposable metered dose inhalers.
A known mechanical dose counter includes an indicator panel carrying marks to indicate the number of doses and which is rotatably supported, and a control lever for rotating the indicator panel. The control lever is urged to return to the initial position by an elastic means such as a spring. When the canister body of the aerosol canister is manually depressed, the downward force on the aerosol canister moves the control lever downward against the urging force of the spring of the control lever. The control lever is sprung back to the original position on release of the downward force on the aerosol canister. The control lever engages the indicator panel and rotates it by a predetermined angle while being depressed or returning to the original position. That is, in one stroke of reciprocal movement, the control lever rotates the indicator panel to increment the display by one.    Reference 1: JP-T-2007-513666    Reference 2: JP-T-2003-512265    Reference 3: U.S. Pat. No. 6,446,627